In a sealed rechargeable cell, such as a nickel/cadmium cell, one of the most important aspects of the cell's cyclic operating mechanism is the generation, transport and recombination of oxygen. This is particularly important during overcharge of the cell since the processes that occur during overcharge of a cell, such as a nickel/cadmium cell, are as follows:
(1) Oxygen is generated at the positive electrode.
(2) The oxygen moves toward the negative electrode along two paths, the first through a gas phase and the second through a liquid phase.
(3) At the negative electrode, the oxygen reacts with Cd to form Cd(OH).sub.2.
The rate of recombination should be equal to or faster than the rate of generation, and gas transport from positive to negative electrode should be fast enough so as not to be a limiting factor in the overall process. Unfortunately, the latter is not usually the case because the oxygen has to travel part of the way through the electrolyte, and the diffusion through electrolyte is a slow process. If a path solely through the gas phase could be provided for, the oxygen recombination reaction would be much enhanced. The basic reactions related to oxygen recombination can be written as follows: ##STR1##
All three reactions either occur at the solid-liquid interface or in the liquid phase. For reaction (2) to occur, oxygen has to diffuse through the liquid phase and it is generally assumed that this oxygen diffusion is the limiting step in the recombination reaction. Oxygen transport through the gas phase is orders of magnitude faster than oxygen transport through the liquid. The transport limitation can be minimized by making the distance through the liquid as short as possible. At the same time it should be realized that dry areas of the electrode do not take part in the electrochemical cell reaction. Ideally then, the electrode should have many areas where the liquid layer on the surface is extremely thin (approximately1 micron).
Polytetrafluoroethylene has been used extensively in electrode applications, especially in the fuel cell area. In most of these applications, however, the polytetrafluoroethylene was sintered and as such it would not provide areas where the liquid layer on the surface would be extremely thin.
U.S. Pat. Nos. 3,630,781 and 3,954,501 disclose the use of polytetrafluoroethylene in depth as a binder in pasted negative electrodes and as an aid to oxygen recombination in both pasted and sintered electrode cells. In addition, these references teach that the polytetrafluoroethylene should not be sintered and that the lower effective level of the polytetrafluoroethylene should be 0.5 percent by weight of the active material and the binder. These references further disclose that when a fluorocarbon polymer is used as an impregnant for nonpasted electrodes, the fibrous form of the polymer permits maximum contact of the active particles with the electrolyte and hence maximum availability. Thus the high degree of hydrophobicity of the fluorocarbon polymers will at the same time render the active material also highly accessible to gas present within a sealed cell during overcharge.
U. S. Pat. No. 3,451,856 discloses the use of nonsintered polytetrafluoroethylene in a fuel cell electrode. Specifically, an electrode having a catalyzed layer is dipped into an aqueous dispersion of polytetrafluoroethylene ranging in concentration from 0.1 percent to 10 percent by weight of the dispersion so as to provide a nonwettable coating for the catalyst layer.
An object of the present invention is to provide on the outer surface only of a coherent powdered electrode a substantially uniform dispersion of polytetrafluoroethylene in an amount less than 0.5 percent by weight of the active material in the electrode, excluding the carrier.
Another object of the present invention is to provide on the outer surface only of a pressed powder cadmium electrode a substantially uniform dispersion of polytetrafluoroethylene in an amount less than 0.5 percent by weight of the active material in the electrode.
Another object of the present invention is to provide a coherent powdered electrode for use in a rechargeable cell wherein the outer surface only of the electrode contains a substantially uniform dispersion of polytetrafluoroethylene in an amount less than 0.5 percent by weight of the active material in the electrode.
Another object of the present invention is to provide a rechargeable cell employing a pressed powder or an impregnated sintered cadmium electrode and wherein disposed on the outer surface only of the cadmium electrode is a substantially uniform dispersion of polytetrafluoroethylene particles in an amount less than 0.5 percent by weight of the active material in the electrode.
The following and additional objects will become more fully apparent from the following description.